This invention relates to the preparation of rigid cellular polyurethanes. This invention further relates to catalyst-containing precursors for these polyurethanes which can be stored for considerable periods of time without any significant decrease in reactivity.
Rigid polyurethane foams are widely used as insulating materials in the construction industry. The light weight and low heat conductivity of these foams make them desirable for use as insulation in refrigerators and containers for hot or cold liquids.
Rigid polyurethane foams can be prepared using a variety of well known methods. In some instances it is desirable to prepare the foam at the location where it will be employed, such as between the inner and outer walls of a building or container. For this type of application it is most preferred to employ a two-component system, one of which is a polyfunctional isocyanate such as polymethylene polyphenyl isocyanate. The second component contains the polyol, gel catalyst and blowing agent together with any modifiers or additives. A surfactant such as a siloxane polymer is usually included to ensure a uniform cell structure in the final foam. The second component is usually employed as a pre-packaged mixture that is prepared weeks or even months before it is reacted with the isocyanate.
It is well known that both divalent and tetravalent tin compounds are effective gel or polymerization catalysts for cellular polyurethanes. Three of the most preferred catalysts are stannous 2-ethyl hexoate, dibutyltin di(lauryl mercaptide) and dibutyltin dilaurate. Unfortunately these catalysts, in addition to most of the other classes of tin compounds, undergo a substantial loss of activity when incorporated into a precursor or "masterbatch" that is subsequently stored for any considerable length of time before being reacted with the isocyanate component. The masterbatch contains one or more of the reactants, catalysts and modifiers which are subsequently combined with the isocyanate component to form the foam product. These tin-containing catalysts are therefore not suitable for use in any of the aforementioned pre-packaged two-component systems for preparing rigid cellular polyurethanes.
It is therefore an objective of this invention to define a class of diorganotin compounds which do not lose catalytic activity in the presence of conventional precursors for rigid polyurethane foams. A second objective is to prepare storage-stable precursors for rigid cellular polyurethanes.